1. Field of the Invention
This invention is related to thermosetting powder coating compositions and particularly to compositions containing epoxy resins.
2. Description of Prior Art
Epoxy resin powder coating compositions which are capable of being crosslinked and various agents which effect the crosslinking are well known. The prior art contains numerous references to compositions which contain epoxy polymers and to conventional crosslinking agents such as anhydrides or amines.
U.S. Pat. No. 3,882,064 issued May 6, 1975 to W. Pregmon, discloses thermosetting powder coating compositions based on an epoxy resin of the epichlorohydrin-bisphenol-A type crosslinked by dicyanidiamide. Similar powder coatings are shown by Nagel, U.S. Pat. No. 3,028,251, issued Apr. 3, 1962; and Parry, U.S. Pat. No. 3,400,098, issued Sept. 3, 1968. Dicyandiamide-cured compositions can be water sensitive and therefore are not well suited to be pipe coatings. No disclosure is made in these patents as to the resistance of their respective compositions to adhesion loss under cathodic protection.
Powder coatings which cure rapidly and have resistance to adhesion loss when subjected to cathodic protection are particularly demanded by the pipe industry. The prior art coatings capable of curing in less than 2 minutes at 200.degree. C. have been commonly found to lose adhesion when subjected to cathodic protection. Pipes are often subjected to cathodic protection, after burial in the ground, by connecting them to the negative terminal of a direct voltage source. It is important that the coating not lose adhesion under these conditions.
U.S. Pat. No. 3,819,564, issued June 25, 1974 to W. Gindrup and A. J. Siegmund, Jr., discloses thermosetting powder coating compositions based on epichlorohydrin-bisphenol-A epoxy resins cured by aromatic anhydrides and hydroxypyridines. This composition is said to cure in 45 seconds at 200.degree.-230.degree. C. and has acceptable resistance to adhesion loss during cathodic protection. However, its flexibility is less than that exhibited by coatings which require more than 2 minutes to cure.
The use of blends of epichlorohydrin-bisphenol-A epoxies and novolac epoxies in powder coating compositions is shown, for example, in U.S. Pat. No. 3,484,398, issued Dec. 16, 1969 to W. I. Childs in which a silica filler and anhydride curing agent were used. These compositions, formulated to serve either a molding or coating function, are deficient with respect to certain desirable coating composition properties, notably cure-time and flexibility, when compared to compositions designed for use solely as coatings.
The use of curing agents other than the conventionally used anhydrides and amines is also part of the art. An agent, believed to be a hydroxy-containing aromatic-aliphatic ether, specially designed for use in curing epoxy resin powder coatings, is shown in Dow Chemical Company Technical Data pamphlet entitled "Dow Experimental Hardener XD-8062," dated Mar. 2, 1975. Some of the disclosed uses for this curing agent are in compositions based on epoxy resins similar to those used in the present invention. These compositions exhibit acceptable flexibility and adhesion but require a relatively long cure time.
Therefore, there remains a need for an epoxy resin powder coating composition which cures rapidly, preferably in less than 2 minutes at commercially acceptable temperatures, and which has good adhesion, adhesion retention under cathodic protection, and flexibility.